1. Field of the Invention
The present invention relates to an image forming apparatus, such as a laser beam printer, a copying machine, and a facsimile machine, and an optional cassette feeder unit provided therebelow.
2. Description of the Related Art
A conventional image forming apparatus will be described using a black and white laser beam printer shown in FIG. 10. The body 52 of the laser beam printer 51 is provided with a cassette feeder 53 integral therewith and serving as a sheet-supply section under the body in FIG. 10. Under the cassette feeders 53, an optional cassette feeder unit 54 is provided. This optional cassette feeder unit 54 is a unit separate from the body 52. The body 52 is positioned and mounted on the optional cassette feeder unit to thereby build the laser beam printer 51.
At the lower part of the laser beam printer 51 on the front surface (right-hand side in FIG. 10), two cassette insertion slots 52a and 54a are formed. Universal cassettes 55 and 56 serving as sheet carrying units can be mounted in the direction indicated by arrows in the figure. Sheets P loaded and stored in the universal cassettes 55 and 56 are fed to a stopping register roller pair 59 by sheet feeding rollers 57 and 58, serving as sheet feeding means.
Luminous flux 61 emitted from a laser oscillating unit 60 according to image information is reflected by a mirror 62 and forms an image on a photosensitive drum 64 provided for a process cartridge 63. The photosensitive drum 64 rotates at a constant speed. A sheet P is carried by the register roller pair 59 at a certain timing to a position between the photosensitive drum 64 and a transfer roller 66 such that a latent image formed on photosensitive drum 64 at point A is developed with toner 65 from developing unit in the process cartridge 63 and the developed latent image is transferred to the sheet P at point B.
After image transfer, the sheet P on which the non-fixed toner image is placed is nipped by a fixing roller pair 67. Heat and pressure are applied to the non-fixed toner image and it is fixed. The sheet P is then discharged to the outside of the apparatus with the image surface facing upward by a convey roller pair 68 and a face-up discharging roller pair 69. Alternatively, the sheet P is discharged at the upper section of the apparatus with the image surface facing downward by the convey roller pairs 68 and 70, and a face-down discharging roller pair 71.
The optional cassette feeder unit 54 is provided with the optional universal cassette 56 and the sheet-feeding roller 58 for feeding and conveying the sheet P.
As shown in FIG. 11, on the upper surface of the optional cassette feeder unit 54 positioning pins 54a, 54b, and 54c and a floating connector (male) 72 are provided. At the bottom of the cassette feeder 53, which is combined together with the body 52, are provided positioning holes 53a (round hole), 53b (oblong hole), and 53c (round hole) which fit with the positioning pins 54a, 54b, and 54c, respectively, and a floating connector (female) 73 which connects with the floating connector (male) 72.
Rubber- or resin-made legs 74 are provided at the bottom of the cassette feeder 53. An extension path 75 is formed in the cassette feeder 53 for conveying the sheet P sent from the optional cassette feeder unit 54, as shown in FIG. 10.
Generally, either of the floating connectors 72 and 73 can freely move on a plane within a range of several millimeters. A connection portion is provided with a taper serving as a guide. To assure positioning precision between the body 52 and the optional cassette feeder unit 54, the positioning pins 54a and 54b and the positioning holes 53a and 53b are configured with relatively high fitting precision.
To connect the body 52 to the optional cassette feeder unit 54, the positioning pins 54a and 54b are first fit in the positioning holes 53a and 53b before the floating connectors 72 and 73 connect. The body 52 and the optional cassette feeder unit 54 may connect in the state in which a part of the bottom surface of the cassette feeder 53 near the floating connector 73 butted up against the upper surface of the floating connecter 72. In this case, the floating connectors 72 and 73 do not connect and an electrical signal from the body 52 is not transmitted to the optional cassette feeder unit 54, causing malfunction.
To prevent this problem from occurring, the positioning pins 54a and 54b are made relatively high in order to positively connect the floating connectors 72 and 73 with the positioning pins 54a and 54b being fit in the positioning holes 53a and 53b. In addition, the positioning pin 54c and the positioning hole 53c are provided near the floating connectors 72 and 73 for facilitating the connection between the connectors 72 and 73. Since the positioning pins 54c and the positioning hole 53c are configured such that they have certain margins in fitting dimensions, the body 52 can be moved slightly against the optional cassette feeder unit 54. This facilitates the connection between the floating connectors 72 and 73, allowing the height of the positioning pins 54a and 54b to be lowered to some extent.
The movement of the sheet P supplied from the optional cassette feeder unit 54 will be described below. The sheet P loaded on the cassette 56 is taken out by the sheet feeding roller 58 and is conveyed to the register roller pair 59 through the extension path 75 in the cassette feeder 53 of the body 52. At the bottom of the body 52, which connects to the extension path 75, a communication slot 76 is formed. The sheet P passes through the communication slot 76.
A locking mechanism is provided for preventing the body 52 and the optional cassette feeder unit 54 from separating or shifting relative to each other after the body 52 is mounted on the optional cassette feeder unit 54. The locking mechanism locks the connection between the body 52 and the optional cassette feeder unit 54 by engaging a locking lever 78 provided on the upper surface of the optional cassette feeder unit 54 with a securing pin 77 provided at the bottom of the cassette feeder 53. At a side of the optional cassette feeder unit 54, a locking release lever 79 is provided for releasing the engagement of the locking lever 78 against the securing pin 77 by rotating the locking lever 78 out of engagement with pin 77.
FIG. 12(a) shows the condition of the locking mechanism immediately before the body 52 is mounted on the optional cassette feeder unit 54. FIG. 12(b) illustrates the condition of the locking mechanism after the body is mounted on the optional cassette feeder unit 54. The locking lever 78 is mounted such that it can be rotated about a rotation shaft 78a and abuts against stopper 81 by the force from pulling spring 80.
When the body 52 is lowered after being positioned with the positioning pins 54a and 54b, the securing pin 77 of the body 52 pushes the taper portion 78b of the locking lever 78 in the optional cassette feeder unit 54 to rotate the locking lever 78 about the rotation shaft 78a clockwise as shown in FIG. 12(b). When the body has been mounted on the optional cassette feeder unit, the locking lever returns to the position indicated with the dotted line having two short dashes and engages with the securing pin 77, completing the connection. To release locking, the locking release lever 79, which is secured to the rotation shaft 78a, is rotated such that the locking lever 78 is rotated clockwise against the force caused by the pulling spring 80.
The conventional image forming apparatus, however, has the following problems. Since the securing pin 77 and the locking lever 78, which are part of the locking mechanism, are provided inside the body 52 or the optional cassette feeder unit 54, it is necessary to allocate space for them, thereby restricting the compactness of the apparatus.
Since the locking lever 78 is rotated and engaged with the securing pin 77, the locking mechanism is complicated, requiring relatively high machining precision for its members. This also requires relatively high precision in assembly, making the apparatus expensive.
To add the optional cassette feeder unit 54 to the body 52, the positioning pins 54a and 54b provided on the upper surface of the optional cassette feeder unit 54 are aligned with the positioning holes 53a and 53b provided at the bottom of the cassette feeder 53 combined with the body, the body 52 is placed on the optional cassette feeder unit 54, and then the floating connectors 72 and 73 provided for the body 52 and the optional cassette feeder unit 54 are connected. This configuration requires that the positioning pins 54a and 54b be long or the positioning pin 54c and the positioning hole 53c be provided near the floating connectors 72 and 73 for facilitating the connection between the connectors 72 and 73.
When the positioning pins 54a and 54b are made long, if the body is relatively light, for example, about 20 kg or less, it is easy to fit the body 52 in the optional cassette feeder unit 54. If the body 52 is relatively heavy, such as that requiring two persons to carry it, it is difficult to fit the body 52 in the optional cassette feeder unit 54. The positioning pins 54a and 54b may hit the body 52 during the fitting. The body may hit the floating connector 72, which protrudes from the optional cassette feeder unit 54, breaking the connector 72.